1. Field of the Invention
This invention relates to time base system synchronization and more particularly to a method and apparatus for synchronizing one or more remotely located local clocks and oscillator frequencies with a master clock and master oscillator frequency.
2. Description of the Prior Art
Community navigation and control systems have been developed which permit a plurality of user aircraft, ships or other vehicles forming an operating community to navigate and be controlled by navigation and guidance signals received from a master or control aircraft, ship, vehicle or ground station. The user members of the community are linked to each other and to the master unit by radio linkages which employ digitally encoded signals for identification, event timing and data transmission. The radio ranging signals permit navigation by trilateration computations and transmit master navigational data from the control unit to each of the user vehicles. Time division multiple access techniques are utilized with the digitally coded radio transmission signals to permit two-way communication between the user members of the community and the controller and also to facilitate identification of the community members. The time base for the system is "real time" and consists of successively repeating system cycles or "epochs". Each system epoch or cycle is of a finite time duration and is divided up into a number of equal time "slots". Each of the user units and the control unit is assigned to a separate and different time slot in the system cycle, so that two-way radio communication between the control unit and each of the user units may be had once during each system cycle during the user's time slot. When the time base of each of the user units is synchronized with the time base of the control unit, the digitally encoded radio signals may be employed to time and sequence events and determine slant range measurements between units. Obviously the accuracy and reliability of systems of this type depend to a large extent upon the degree of synchronization between the clock systems of the user units and the clock system of the control unit. Since the members of the community may include aircraft, ships and other movable vehicles, a common power supply may not be employed and synchronization of the clock systems of the units must be accomplished by radio signals. The synchronization technique employed must not only be highly accurate and reliable but must also lend itself to mechanically-rugged mechanization which is suitable for use in land, sea and air environments.
Apart from use in community navigation systems, time base synchronization systems are often employed to synchronize the clock systems of a plurality of widely-separated computers and/or industrial control systems to permit joint computational use and/or joint industrial control applications. Furthermore, time base synchronization is used in purely horological applications to permit one or more remotely-located visual display clocks to be synchronized with a master clock to provide accurate and uniform visual time display at all locations.
Prior art time base synchronization systems generally make the time base correction by resetting or slaving the local time base to make the measured time error zero within the quantization level of the measurement. This technique makes the accuracy of such systems directly dependent on the accuracy of the discrete time error measurement which is, of course, corrupted by noise and degraded by system errors. Since the time synchronization error is measured instantaneously, it is subject to large deviations in steady state condition because of the inherently short time constant involved and because the past history of deviations and time error corrections are not reflected in the correction signal resulting from the instantaneous time error measurement. Such a system does not have the advantage of averaging multiple time error measurements to produce a time error signal which is dependent upon the rate-of-change of time synchronization errors over a short term history. Additionally, in the prior art systems, frequency corrections are generally not made to the local oscillators which produce the basic clock rate, with the result that the quality and accuracy of the generated local time base is very dependent upon the quality of the local oscillators employed, so that the use of relatively inexpensive, commercially-available oscillators of poor quality is precluded. Some of the known time base synchronization systems utilize frequency and time standard radio signals which are broadcast from National Bureau of Standards radio stations. Comparison receivers are used to compare the received frequency and time signals with the frequency and time of local time base systems to produce an error signal which is then employed to bring the local systems into synchronization with the transmitted frequency and time standards. These systems are dependent for their accuracy on a long time integration which requires very large averaging times. The equipment required is complex and expensive and is usually suitable for use only in a controlled laboratory environment.